1. Field of the Invention
This invention relates to an exposure apparatus for manufacturing a semiconductor integrated circuit and, more particularly, to a mechanism for controlling the position of a stage on which a photosensitive substrate, formed of a semiconductor wafer or the like, is placed, and where the stage is moved two-dimensionally.
2.Description of the Related Art
A step and repeat type of reduction-projection exposure apparatus (stepper) is widely used for a lithography process in the manufacturing of semiconductor integrated circuits. There are two types of alignment systems for positioning between a projected image of a circuit pattern formed on a reticle, and a rotating pattern (hereinafter referred to as a "chip") previously formed on a photosensitive substrate (hereinafter referred to as a "wafer") in this type of stepper. The first type is an on-axis alignment system for performing positioning by detecting a mark on the reticle and a mark on the wafer through a projection lens and the reticle. The second type is an off-axis alignment system for detecting only a mark on the wafer without detecting a mark on the reticle.
In a stepper having the off-axis alignment system, a mark detection reference position for the alignment system (hereinafter referred to as "alignment position") and a projected position of the projected image of the reticle circuit pattern (hereinafter referred to as "exposure position") differ from each other. Therefore, in the case of an apparatus, such as that shown in FIG. 5, having an alignment system 103, located on a measuring axis of one laser interferometer 105 and off a measuring axis of another laser interferometer 104, an alignment mark of a wafer 101 is detected by using the alignment system 103. The alignment position of the corresponding chip in the directions along X- and Y-axes is read from the laser interferometers 104 and 105 by measuring the extent of movement of mirrors 107 and 108, which are fixed on a stage 109. However, a measurement value detected in this manner includes an Abbe error, since the alignment position is not located on the measuring axis of the laser interferometer 104. According to the conventional method, the extent of movement of the mirror 108 is measured again by a uniaxial laser interferometer 106, the amount of rotation, i.e., the amount of yawing of the stage 109 is calculated in a controller (not shown) from the difference between measurement values obtained by the laser interferometers 105 and 106. The measurement value obtained by the laser interferometer 104 is corrected by using the amount of yawing that was calculated. The stage 109 is only moved through a base line (a relative positional relationship between the alignment position and the exposure position) on the basis of the corrected value. Also, when the wafer 101 is transported to a position below the projection lens, chip positioning is performed by considering the amount of rotation of the stage 109 so that the projected image of the circuit pattern and the chip is accurately superposed, and exposure is thereafter effected.
However, as shown in FIG. 6, if the perpendicularity between the mirrors 107 and 108 is changed by thermal deformation, or the like, of the stage 109 in the above-described conventional exposure apparatus, a positional error "e" corresponding to a change in the perpendicularity occurs as a base line error when the stage is moved from the alignment position to the exposure position. The circuit pattern superposition accuracy is thereby reduced.